Help with simple voltage/current sensing relay.

[Roff]

The resistor divider on the NPN's gate lowers the 4.6 volts bellow 500mv, the base junction never becomes forward biased. If you skip the 10k and 1k resistor divider on the base and just use a 1k resistor to the base it works fine. What's the resistor divider for?

Yeah, I had a brain fart. The base-emitter resistor should be 10k, not 1k. I like base-emtter resistors where the signal might be noisy, or the input pull-down is unknown.
I'll edit my schematic when I get downstairs to my other computer.

 

[Sceadwian]

hrmm, didnt' notice that (didn't simulate it either) don't worry about modifying yer schematic, the one I modified is still yours even if I posted it =)

 

[Roff]

Uncle's last schematic should work just fine, given what you've stated so far I don't think the 470 ohm gate resistor is NOT required though, use it at first remove it if it doesn't work. The voltage isn't incredibly sensitive, generic NPN/PNPs will be well within a car voltages range.

It won't work. See post #60.

 

[Sketch_hs]

So...


Which one will work...?

 

[Roff]

So...


Which one will work...?

See my corrected schematic in post 52.
Sceadwian's circuit in post 54 should work if you reduce the value of the 600 ohm resistor to 100 ohms or less (it is not really necessary, same for the 100 ohms in my circuit). IMHO, it wastes a little power, which probably is not a problem in a car.

 

[Sketch_hs]

well i made these two: https://www.electro-tech-online.com/attachments/relaydrvr-jpg.28937/

And couldn't get either to switch a small relay with supplied signal from AC amp...

not sure why

 

[Sketch_hs]

any more input?


I found what seems to be some opto-isolators. Anything in particular i need to use them as a switch?

 

[Sketch_hs]

What kind of opto isolator will switch on with the 4.6v and allow the 12v through the other side?

 

[Roff]

So...


Which one will work...?

Why have you ignored the answer to this question? If it won't work, kindly tell us why.

 

[Sketch_hs]

Why have you ignored the answer to this question? If it won't work, kindly tell us why.

Do the circuits posted allow the ecu to pull all the power it needs?

I tried giving it 12v through some resistors and it would stop engaging the compressor at about 600ohms from a direct ~14v source.

It needs about 20-25 mA to kick on (it's 13.6-14.0v when car is running) and holds solid at 20.66 mA once engaged.

If that circuit will work i'll make it. I have all the stuff

 

[Roff]

Do the circuits posted allow the ecu to pull all the power it needs?

I tried giving it 12v through some resistors and it would stop engaging the compressor at about 600ohms from a direct ~14v source.

It needs about 20-25 mA to kick on (it's 13.6-14.0v when car is running) and holds solid at 20.66 mA once engaged.

If that circuit will work i'll make it. I have all the stuff

Either one should work if you follow the recommendations in my previous post.
Of course, the relay circuit you posted (with the BJT) should have also worked.

 

[Sketch_hs]

yeah i don't know why it didn't work!

i'll try the one in post 52... any way I can test that the circuit it working with a multimeter? (regardless of whether it works for the ECU)

On a side note, i found a small LED in an old broken printer and the signal (4.6v) from the ECU was enough to make it light up steady.

 

[unclejed613]

yep, you're right........put a 470 ohm resistor between the collector of the NPN and the base of the PNP.....

 

[Sketch_hs]

In place of the 3.9k one?

 

[Sketch_hs]

Anything else before I make it? Won't be home for 2 hrs thankfully got my blackberry lol

No more soldering iron burns I hope

 

[Roff]

In place of the 3.9k one?

No, he was talking about his circuit, where he forgot to put in a current limiting resistor. 3.9k is fine. 470 ohms is a waste of power.

 

[Sceadwian]

The last circuit ROFF posted should work just fine as is, if it doesn't then verifiy you're soldering everything together right.

 

[Sketch_hs]

IT WORKS IT WORKS IT WORKS!!!!!!


Thank you!

 

[Sceadwian]

Congratulations =) Sorry for the run around.

 

[Sketch_hs]

Thank you guys so much! It's a thing of beauty. You press the A/C button in the car and the idle raises up and the compressor clutch engages.


You've actually found a solution for MANY people who are having this problem with this popular swap.

In case anyone was wondering, it's a 95 Toyota Supra, non turbo chassis with a Japanese Turbo motor swap, US wiring harness, and a Japanese ECU.

Even 600 wheel horse power isn't fun if the AC doesn't work when it's 100 degrees out!



On a side note, I learned a lot figuring this out and am itching for another project... ideas?